State-of-the-art-military armor systems for vehicular and personnel (body armor) protection frequently make use of lightweight, very high compressive strength ceramics such as silicon carbide (SiC), boron carbide (B4C) or alumina as the so-called “strike face” of an armor laminate package. The purpose of the strike face material, as typically employed in high performance ceramic composite armor systems, is to blunt and defeat incoming metallic (often armor-piercing) projectiles by overmatching the compressive properties of the incoming projectile during the early (compressive shock) portions of the impact event. High modulus, high strength ceramics can easily have four to five times the dynamic compressive strength of projectile materials such as steel, tungsten or tungsten carbide. Thus, it is possible to shock the incoming projectile to the extent that compressive fracture is initiated. This decreases the ability of the projectile to defeat the armor system. Additionally, the use of high elastic modulus strike face materials also facilitates radial load spreading of the compressive shock front at the projectile/armor interface; this phenomenon allows lateral engagement of the ceramic to take place, promoting formation of a radially-expanding pulverized (comminuted) zone ahead of and around the impact interface. The combination of load spreading and attendant formation of a comminuted zone comprised of failed ceramic promotes mushrooming of the incoming projectile head and decelerates the projectile as it transits through the ceramic, reducing the intact areal momentum of the projectile and ensuing ceramic fragments. When ceramics are employed in laminate constructions and are backed with high tensile strength, high-toughness “momentum trap” composites such as Kevlar or Spectra fibers, very mass-efficient armor systems can be designed. The mass efficiency of such ceramic composite armor systems is generally two to five times higher than that associated with high hardness steel or similar high strength metallic armor plate.
Over about the past twenty years, it has been discovered that the ballistic performance of ceramic armor is critically dependent on the specific design attributes and geometrical configuration of the entire armor system. In particular, it has been observed that enhanced destruction and fragmentation of an incoming projectile can be obtained by increasing the so-called “dwell” time of the projectile on the front face of the ceramic armor during the very early stages (the first 5-10 microseconds) of the impact event (see Hauver, G. E. et al, L. J., 1994, “Enhanced Ballistic Performance of Ceramics,” 19th Army Science Conference, Orlando, Fla., June 1994, pp. 1633-1640). In general, the longer the dwell time on the front face, the more completely the projectile can be attenuated and fragmented. Enhanced dwell time on the front face of the ceramic armor leads to a phenomenon that is called interface defeat, wherein the projectile face mushrooms radially outward without significant penetration in the thickness direction; this increases the projectile frontal area and thus decreases its subsequent ability to core a cylindrical plug out of the ceramic armor.
The phenomenon of dwell is used to particular advantage in medium or heavy ceramic armor systems that are intended to defeat larger caliber (12.7 mm and above) high kinetic energy projectiles. It has been found that physical confinement of ceramics such as B4C, SiC or TiB2 delays the lateral and axial spreading of the comminuted zone ahead of the projectile, thus increasing the ballistic efficiency of the ceramic. Physical confinement of ceramic armor tiles can be performed by a number of means, such as by shrink-fitting ceramic tiles or bricks into metallic containers, or by other bonding methods involving the use of welded, bolted, brazed or adhesively bonded metallic containers. Interestingly, for relatively thin armor tiles (less than 0.4-0.5″ thick), it has also been found that light lateral or hydrostatic confinement can be of benefit in delaying the flexural failure of armor tiles on the rear face away from a projectile; this effect can also be used advantageously to increase the ballistic efficiency of ceramic armor-based protection systems.
However, ceramic armor is not without serious engineering and practical shortcomings. High hardness, high elastic modulus ceramic materials such as SiC and B4C are very brittle and have poor durability and resistance to dropping or even rough handling under typical field conditions. Furthermore, the low toughness of high performance ceramics implies that essentially all armor-grade ceramics have poor multiple hit capabilities. Once a large ceramic tile such as a torso plate is impacted with a high velocity rifle round, the subsequent impact response of the armor is seriously compromised. This complicates effective tactical employment and packaging of the ceramic armor because additional composite layers which surround the ceramic have to be especially engineered to contain spill fragments, while also limiting adjacent crack damage to the maximum extent practical. Such measures add cost and weight to ceramic armor systems while not significantly enhancing ballistic performance.
In view of the above, there is a clear need to improve the impact resistance, ballistic efficiency and structural integrity of ceramic armor now employed on a widespread basis in many types of armor systems. One relatively obvious and popular method to overcome the disintegration of ceramic armor is to encapsulate a ceramic armor with a layer of surrounding metal. In the past, such layers have been formed on or around ceramic cores or tiles by techniques such as powder metallurgical-forming, diffusion bonding, and vacuum casting of liquid metal layers.
U.S. Pat. No. 4,987,033, for example, teaches methods for metallic encapsulation of ceramic cores with powdered metal layers that are cold isostatically pressed, vacuum sintered and then hot isostatically pressed to final density. These methods have severe shape limitations, involve the use of relatively costly cold isostatic press tooling, require a complicated and costly multiple step processing sequence, and still require complicated and costly post-machining to produce a metallic encapsulating layer with consistent areal density (which is required for armor system design).
U.S. Pat. Nos. 3,616,115 and 7,069,836 respectively, teach methods for metallic encapsulation of ceramic armor based on vacuum hot pressing and/or diffusion bonding of ceramic tiles and metallic stiffening layers into machined arrays of lattice-type metallic frameworks. While capable of producing well-bonded and geometrically-consistent metallic encapsulation layers, these methods are also costly and very limited with regard to their shape-forming capability and the related ability to be transitioned to large-scale manufacturing, as they require expensive restraint tooling and die sets that essentially limit vacuum hot press or die pressing-based diffusion bonding to flat plate geometries.
Modifications of conventional liquid metal casting processes have also been used as in U.S. Pat. No. 7,157,158. These methods, while capable providing for encapsulation of different ceramic materials as well as complex shapes, require complex and costly molds, and the casting process itself presents many challenges since most metals of interest for encapsulation (Al, Mg, Ti etc.) shrink anywhere from about 3 to 12% upon solidification. The high coefficient of thermal expansion relative to armor-grade ceramics such as silicon carbide, boron carbide or alumina frequently leads to liquid metal casting-based encapsulation results generating very high stresses around the ceramic core—which can easily result in the fracturing of the ceramic being encapsulated. (See Wells, J. M. et al, “Pre-Impact Damage Assessment Using X-Ray Tomography of SiC Tile Encapsulated in Discontinuously Reinforced Aluminum Metal matrix Composite,” ACUN-3 International Composites Conference, February 2001, Sydney, Australia.) This situation would also be worsened for more complex ceramic armor tile geometries, such as would be the case for a body armor torso plate.
Thus, there are still deficiencies with the metallic encapsulation of ceramic cores in the prior art. There is a need to develop metallic encapsulation methods which are less complicated, less costly, capable of working with a wide range of metal and ceramic materials combinations, and also compatible with the requirements of reproducible and large-scale manufacturing. In view of the previous limitations concerning metallic encapsulation of high performance ceramics to produce armor, the objectives of our invention are as follows:                Enhance multiple hit resistance via metallic encapsulation with metallurgically-bonded layers of metal surrounding ceramic armor tiles and improve the durability and damage tolerance against physical abuse and routine handling for ceramic armor elements by providing a robust metallic container for individual tiles or tile arrays.        Enhance hydrostatic confinement to increase dwell time for a projectile on the front face, thus promoting mushrooming and defeat of anti-armor projectiles ranging from rifle rounds to high velocity kinetic energy-based anti-tank long rod projectiles.        Provide for tailorable interfacial bond strength ranging from shear strengths of a <5 MPa to >200 MPa via use of measures such as metallic foil interlayers, thin metallic films, solders, or eutectic-forming braze layers.        Employ standard, low-cost methods for manufacturing encapsulating layers based on sheet metal and similar metallurgical forming methods and provide methods that are amenable to metallic encapsulation of complex ceramic armor shapes such as torso plates, vehicular door panels or armor vehicle subcomponents.        Employ highly reproducible methods for diffusion bonding of hundreds or thousands of metallically encapsulated armor tiles in one bonding run, thus significantly enhancing process reproducibility and statistical ballistic response behavior while also reducing unit costs and form metallic layers with highly reproducible and tailorable areal density for weight-sensitive armor systems.        Provide a method for working with a wide range of encapsulating materials such as titanium, aluminum or magnesium; realizing a reduction of system weight and cost as a result of the having the option hermetically to encapsulate ceramic tiles with widely available, low-cost metals.        Provide a method for manufacturing individually encapsulated ceramic tiles or tile arrays that can be welded, brazed, mechanically affixed or otherwise bonded to other structural elements or other support members such as would be found on a land vehicle, boat or ship frame, thus simplifying installation and replacement procedures for high performance armor in field settings.        Provide a method for the manufacture of metallic encapsulated ceramic armor with superior corrosion resistance via use of metals such as Grade 2 titanium, alpha or beta titanium alloys, and/or suitably chosen Al or Mg alloys, thus permitting advantageous use of such armor in marine or similar corrosive atmospheric conditions.        